1. Field of the Invention
The present invention relates to a semiconductor device, more particularly to a semiconductor device having a barrier film for preventing penetration of moisture from the outside.
2. Description of the Related Art
In the microscopic MOS transistor field of the present day, a secondary slow trapping phenomenon has become a problem. (In the following description, a slow trapping means a secondary slow trapping. Refer to a bibliography "N. Noyori et al: Secondary slow trapping--A new moisture induced instability phenomenon in scaled CMOS devices, 20th Ann. Proc. International Reliability Physics Symposium, pp. 113-121, 1982.").
Slow trapping means a phenomenon in which a characteristic such as V.sub.t of a transistor undergoes a time elapse change caused by moisture contained in interlayer insulation films. As a countermeasure therefor, a film such as a nitride film having a good moisture-proof property is formed as a barrier to prevent penetration of moisture from the outside to avoid generation of slow trapping.
A prior technique will be described with reference to FIG. 1 and FIGS. 2(a), 2(b).
When a barrier film such as a nitride film is formed, for example, if nitride film 31 is formed after wiring process is finished as shown in FIG. 1, incidentally other interlayer insulation films (41, 82, 92, 102, 112 in FIG. 1) made of such as oxide films are thickly provided between silicon substrate 11 and nitride film 31, thereby producing slow trapping caused by moisture included in the interlayer insulation films. Although four layer wiring is shown in FIG. 1, more multilayered wiring will be required as miniaturization and high integration of the wiring proceeds in future, and hence forming the barrier film after finishing the wiring process becomes meaningless as a countermeasure for preventing slow trapping.
If nitride film 31 is formed after forming a transistor as shown in FIG. 2(a), there exists no other interlayer insulation film between silicon substrate 11 and nitride film 31, however in this case, since nitride film 31 is formed directly on silicon substrate 11 in a region of a source.cndot.drain diffusion layer, there occurs a problem such that an increased leak current flows due to a stress produced at the time of forming the nitride film or due to a level generated on a silicon interface.
Therefore, as shown in FIG. 2(b), such a method is now applied which includes the steps of forming underlay oxide 21 for stress relief after forming a transistor, and then forming nitride film 31 thereon. In place of forming nitride film 31, there is another method for forming an underlay oxide, and then applying nitrogen ion implantation thereon to nitrify an oxide surface as a barrier film.
Next, the prior technique will be described with reference to FIG. 3-FIG. 7.
As shown in FIG. 3(a), underlay oxide 21 is first formed for stress relief on silicon substrate 11 which has a semiconductor element formed thereon. Although the required thickness of underlay oxide 21 varies depending upon the thickness of a nitride film to be formed thereon, the thickness of underlay oxide 21 is enough for fulfilling a role of stress relieving, if it is in a range of 100 to 500 .ANG.. Further on underlay oxide 21, nitride film 31 of 50 to 500 .ANG. thick is formed as a barrier film for checking the penetration of moisture. There is another method comprising the steps of, instead of forming nitride film 31, first forming an underlay oxide, and then performing nitrogen ion implantation to nitrify the oxide surface for making it the barrier film.
Then, base interlayer film 41 for flattening the base is formed in a thickness in the range of 8,000 to 15,000 .ANG..
Then, opening contact 51 is provided as shown in FIG. 3(b), and in order to set up an ohmic junction for the contact, an N type dopant and a P type dopant both of high density are injected to the contact on an N type diffusion layer and to the contact on a P type diffusion layer, respectively. However in this case, injection of the dopant immediately after forming the opening contact sometimes induces a lattice defect appearing on the silicon surface, thereby causing trouble such as a leak.
Therefore, as shown in FIG. 4(a), the above high density contact injection is performed after forming protection film 61 of silicon substrate 11. In this case, protection film 61 is formed to 100 to 300 .ANG. thick with such as a plasma CVD oxide. However, since the plasma CVD oxide has poor coverage, the film thickness becomes thin in the contact side wall, and the oxide is hardly formed particularly at the side wall near the bottom of the contact.
Next, as shown in FIG. 4(b), protection film 61 is removed by anisotropic etching.
Then, in order to lower the contact resistance by removing the natural oxide on the silicon, wet etching is applied to the oxide as shown in FIG. 5(a). As for oxide wet etching, it is performed for about 30 seconds with an etching liquid prepared by adding NH.sub.4 F as a buffer to a solution of H.sub.2 O:HF=30:1 (hereinafter this etching liquid is described as 130 BHF). At this time, since almost no protection film 61 exists on the side wall of the contact bottom, the side wall of the contact is etched, but nitride film 31 is not etched and hence eaves 32 of the nitride film is formed inside contact 51. With the above etching time, the protrusion length of eaves 32 of the nitride film is about 300 .ANG..
Next, as shown in FIG. 5(b), although barrier metal 71 is formed by spattering to prevent the reaction between the wiring metal or the contact filling metal and the silicon substrate, no barrier film is formed at this time by sputtering on the under part of aforementioned nitride film eaves 32.
FIG. 6 is a typical view showing an inside state of the contact before barrier metal is sputtered. In other words, when the contact depth is A, the contact diameter B, the underlay oxide thickness C, and the protrusion length of the nitride film eaves D, it is understood that the following inequality is established. EQU tan.sup.-1 (B/A)&gt;tan.sup.-1 ((B-D)/(A-C))
For example, when it is assumed that the contact depth A=8550 .ANG., the contact diameter B=5000 .ANG., the underlay oxide thickness C=500 .ANG., the protrusion length of the nitride film eaves D=300 .ANG., the following data are obtained satisfying the above inequality. EQU tan.sup.-1 (B/A)=0.5292 EQU tan.sup.-1 ((B-D)/(A-C))=0.5285
When miniaturization further advances and the contact size is more miniaturized in future, this tendency will further be increased.
In the conventional example which satisfies the above relational formula, eaves shade 33 formed by nitride film eaves 32 is also produced on the silicon substrate of the contact bottom as shown in FIG. 5(b), and barrier metal 71 is not sputtered on this portion thus leaving silicon substrate 11 as it is exposed.
After then, first layer wiring 81 metal is formed as shown in FIG. 7(a), or filling metal 52 for filling up the inside of the contact is formed as shown in FIG. 7(b). At this time, since nitride film eaves shade 33 portion on silicon substrate 11 on the contact bottom has no barrier film 71 formed thereon while having the silicon remained as it is exposed, the first layer wiring 81 metal or contact filling metal 52 and silicon substrate 11 react upon each other in the following heat treatment process or the like. For example, if the first layer wiring 81 metal is aluminum or aluminum alloy, the silicon and the aluminum react to generate an alloy spike thereby causing a leak. Also for forming filling metal 52, the gaseous phase reaction is utilized using WF.sub.4 gas to form tungsten W. However in this case, silicon and fluorine F react upon each other at the area having no barrier film, and accordingly hollow disfigurement 34 is produced on the silicon substrate at the bottom of the contact thereby inducing a cause of a leak or the like.
An example of means for eliminating the nitride film eaves which causes the above trouble is disclosed in J. P. A. Gazette 208367/1991. The nitride film of this example is formed as a dielectric film of a polysilicon capacitor, and as shown in this example, if only the nitride film of the necessary part is left and other nitride film in the contact opening is removed, no nitride film eaves is produced. However, this method has several problems and is not practical because it requires the complex processes of the increased number and when the nitride film is used as a moisture barrier film and if the nitride film removed area becomes large, it can no more function as the barrier film.
Namely, the prior technique described above has the following problems.
Since the nitride film eaves shade portion is produced on the silicon substrate, no barrier metal is spattered on the nitride film eaves shade portion inside the contact and the silicon substrate is left as it is exposed to directly contact with the wiring metal or the contact filling metal, thereby producing defects which become the cause of the leak or the like deteriorating the reliability of the device.